Titanium oxide is a compound widely used in various industrial fields. The uses are varied, and they depend in particular on its crystallographic structure and its morphology.
Various methods of preparation are known in the prior art. Preparation via a hydrothermal route has been widely explored, but its main drawback lies in the relatively high temperatures and pressures that are required. This is because rutile is the thermodynamically stable phase and its formation requires hard conditions, that is to say acid media, high temperatures and/or long aging times. These hydrothermal syntheses consist in heating, between 140° C. and 1200° C., a precursor such as TiCl4 (H. Yin, Y. Wada, T. Kitamura, S. Kambe, S. Murasawa, H. Mori, T. Sakata, J. Mater. Chem., 2001, 11, 1694) or Ti(OiPr)4 in aqueous medium (C. C. Wang, J. Y. Ying, Chem. Mater., 1999, 11, 3113) or an organic (alcoholic) precursor (S. T. Aruna, S. Tirosh, A. Zaban, J. Mater. Chem., 2000, 10, 2388) in the presence of other reactants (acids, complexing agents, salts, etc.). The particles obtained are generally elongate and their size is of the order of 100 nm. The addition of mineralizing agents (for example NaCl, NH4Cl or SnCl4) has the effect of reducing the size of the rutile particles (H. Cheng, J. Ma, Z. Zhao, L. Qi, Chem. Mater., 1995, 7, 663).
Titanium oxides have also been prepared by hydrolyzing a TiIV compound in aqueous medium at temperatures below 100° C., but the compounds obtained are anisotropic (S. Yin, H, Hasegawa, T. Sato, Chem. Lett., 2002. 564).
It is also possible to obtain TiO2 by electrochemical synthesis, but the synthesis conditions are demanding and the morphology of the compound obtained is difficult to control.
The hydrolysis of various precursors has also been used for the preparation of TiO2. For example, TiO2 is obtained in brookite form from an aqueous TiCl3 solution at a pH below 5 (B. Othani, et al., Chem. Phys. Lett., 1995, 120(3), 292). TiO2 is obtained in the form of a mixture of rutile, brookite, Ti6O11 and Ti7O13 by hydrolysis of an aqueous TiCl3 solution that contains urea, the pH of the solution thus being returned toward basic pH values as the urea decomposes (A. Ookubo, et al. J. Mater. Sci., 1989, 24, 3599). The preparation of TiO2 in rutile form by direct oxidation of TiCl3 at room temperature is described by F. Pedraza, et al. (Phys. Chem. Solids, 1999, 60(4), 445). The method consists either in leaving TiCl3 in water for a certain time (for example 60 hours), in order to hydrolyze the TiCl3 to TiO2, or in heating the aqueous TiCl3 solution to 80° C., then in filtering the particles formed and in drying them at 120° C. or higher. According to J. Sun, et al. (Huazue Xeubo, 2002, 60(8), 1524), a rutile nanopowder is obtained by direct hydrolysis of TiCl3 solutions under mild conditions, in the presence of (CH3)4NOH acting as precipitating agent. The rutile particles are in the form of needles. According to M. Koelsch, et al., (Thin Solid Films, 2004, 86-92, 451-542), the three TiO2 polymorphs may be synthesized by thermolysis of TiCl4 or TiCl3 in aqueous medium, and by controlling the precipitation conditions (acidity, the nature of the anions, ionic force, titanium concentration, etc.) it is possible to control the crystal structure, the size and the morphology of the particles. Thus, spheroidal nanoanatase, pure brookite platelets, of nanoscale dimensions, and rutile of different forms may be obtained. The particular cases illustrated result in the formation of particles of spherical anatase, rutile with a rod or needle morphology, the rods or needles being of various sizes, and platelets of pure brookite. The hydrolysis of TiCl4 in water at a temperature between 20 and 95° C. and an aging time of longer than two days results in rutile (Li, Y. Fan, Y. Chen, J. Mater. Chem., 2002, 12, 1387). The hydrolysis of Ti(iPr)4 in an HCl-acidified aqueous solution at a temperature between 25 and 200° C. results in rutile rods (S. Yin, H. Hasegawa, T. Sato, Chem. Lett., 2002, 564). The hydrolysis of TiOCl2 in HCl or in water containing NH4OH at a temperature of 60° C. gives rutile (D. S. Seo, J. K. Lee, H. Kim, J. Cryst. Growth, 2001, 223, 298).
Thus, it is apparent that, for a given type of method, the particular processing conditions have a major effect on the crystallographic structure and the morphology of the titanium oxide obtained.